1. Field of the Invention
The present invention relates to electrostatographic color printing machines and, more particularly, to opposing corona wire chargers placed in the receiver path after the fusing process within a color printing apparatus.
2. Description Relative to the Prior Art
Commercial reproduction apparatus include electrostatographic process copier-duplicators or printers, inkjet printers, and thermal printers. Such reproduction apparatus, pigmented marking particles, ink, or dye material (hereinafter referred to commonly as marking or toner particles) are utilized to develop an electrostatic image of information to be reproduced on a dielectric (charge retentive) member for transfer to a receiver member or directly onto a receiver member. The receiver member bearing the marking particle image is transported through a fuser device where the image is fixed (fused) to the receiver member, for example, by heat and pressure to form a permanent reproduction thereon.
Commonly, a primary charging device is used to uniformly place a charge on a dielectric member prior exposing the dielectric member to an imaging light pattern. Corona charging devices can serve as the primary charging devices, such as one or more parallel thin wires to which high voltage is applied, a housing partially surrounding the wires and open in a direction facing a dielectric member surface, and an electrically biased grid. A conductive housing is used for DC charging and an insulating housing is typically used for AC charging. A grid includes a metallic screen or mesh, mounted between the corona wires and the dielectric member, and is DC-biased for both DC and AC charging. The grid improves voltage control for the voltage that a primary charger imparts to the dielectric member. A grid also gives a resultant dielectric member voltage uniformity that is generally better than without a grid.
Corona wires having a high DC voltage applied to them can asymptomatically approach a cut-off voltage equal to the DC grid bias plus an overshoot voltage determined by grid transparency, grid/dielectric member spacing and corona voltage. This cut off voltage depends upon the amount of the time it takes for the moving dielectric member to pass under a gridded charger. If this time is longer than a characteristic time constant given by the product of the effective charging resistance and the capacitance of the dielectric member under the charger, the voltage on the dielectric member will asymptomatically approach the cut-off voltage. For tight grids (relatively low transparency) the cut-off of the charging current is very close to the grid bias; that is, the overshoot is small. Conversely, for open grids (relatively high transparency) the overshoot can be significant. Typically, grid overshoot is in the range 100-200 volts, depending on the grid to dielectric member spacing, with smaller overshoots for larger spacings.
In charging systems employing high voltage AC charging waveforms riding on low voltage DC offsets to charge corona wires, the cut-off voltage is generally close to the grid bias and is only weakly dependent on the grid transparency. The actual cut-off voltage is determined by the relative efficiencies of negative and positive corona emissions during the negative and positive AC voltage excursions. Moreover, a high duty cycle trapezoidal AC waveform can be used, as disclosed in U.S. Pat. No. 5,642,254 (issued Jun. 24, 1997, in the names of Benwood et al). In this patent, the cut-off voltage is also dependent on duty cycle, and the cut-off voltage steadily approaches a DC value if duty cycle is steadily increased from 50% (conventional AC) to 100% (DC).
A variety of gridded chargers are presently used in typical reproduction apparatus engines. Examples of grid designs include a continuous wire filament wound back and forth across a charger opening, grids (typically photoetched) mainly composed of thin parallel members that run parallel to or at an angle to the corona wire(s), and hexagonal opening mesh pattern grids. These different types of grids are applied in various types of corona chargers, for example, single or multiple corona wire chargers, pin corona chargers, chargers with insulating or conducting housings, and chargers that use AC or DC corona voltage. There are grids that are planar and grids that are curved to be concentric with a drum dielectric member.
Currently, there are a number of prior art systems that regulate the voltage of a corona wire purely by regulating the current. These current regulated prior art systems can, inadvertently, allow the corona wire voltage to increase to critically high values when a receiver element is between the two chargers. Furthermore, systems that employ current regulation of corona wire voltage can also have voltages vary when different receiver elements are used because of the difference in receiver resistivity. Additionally, current regulated systems can also have arcing develop between the opposing corona wires when a highly resistive sheet exits the charger. This can happen before the current regulation control of the power supply can reduce the output voltage of the supply to react to the change in resistance between the corona wires. Arcing results in undesired electrical noise radiated into the control system of the machine and, possibly, to the environment around the machine. Arcing can also be damaging to the machine hardware and materials.
Other prior art systems employ pure peak-to-peak voltage regulation that allows the current potentially to reach critical, high levels when the interframe is in between the two chargers. In this mode the charger will be operating at an unnecessarily high power level and generate excessive heat in the power supply. Corona wire emissions and the resulting chemical emissions will also be unnecessarily high.
From the foregoing, it should be apparent that there remains a need for a power regulation system of corona wires that can avoid the shortcomings of the prior art and provide a solution that prevents arcing and over-current loading for sheet fed applications.
The present invention is a high voltage power supply for electrostatically discharging prints from a sheet fed printing machine that addresses the prior needs for a power regulation system that can charge corona wires while preventing arcing and over-current loading for sheet fed applications. The power supply has two high voltage outputs that are RMS current regulated and peak-to-peak voltage limited. The current regulation provides a benefit for highly resistive receiver sheets. However, there is a potential for excess voltage that results when using highly resistive receiver sheets, which is corrected by voltage limiting. Each corona wire is connected to one of the two high voltage outputs of the high voltage power supply. The current flow through the ionized air neutralizes and reduces the electrostatic charge in the receivers to uncritical values.
These and other objects of the invention are provided by a power supply for driving opposing corona chargers comprising: a pair of transformers on the power supply, each of the transformers providing an output; a current sense element attached to each of the transformers; a current regulation circuit that is responsive to each of the current sense circuits in accordance with a predetermined parameter to adjust current flowing through the transformers; a voltage monitoring circuit for each of the transformers; and a voltage control circuit that is responsive to the output voltage monitoring circuit to limit the transformer voltage to less than a predetermined value.